THE STAR FORMATION NEWSLETTER An electronic publication dedicated to early stellar evolution and molecular clouds No. 72 — 22 September 1998 Editor: Bo Reipurth ([email protected]) Abstracts of recently accepted papers Correlation Between Gas and Dust in Molecular Clouds: L977 Jo˜aoAlves1, Charles Lada1 and Elizabeth Lada2 1 Harvard–Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA 2 Astronomy Department, University of Florida, Gainesville, FL 32608, USA E-mail contact: [email protected] We report observations of the J = (1–0) C18O molecular emission line toward the L977 molecular cloud. To study the correlation between C18O emission and dust extinction we constructed a gaussian smoothed map of the infrared extinction measured by Alves et al. (1998) at the same angular resolution (5000) as our molecular–line observations. This enabled a direct comparison of C18O integrated intensities and column densities with dust extinction over a relatively large range of cloud depth (2 < AV < 30 mag) at 240 positions inside L977. We find a good linear correlation between these two column density tracers for cloud depths corresponding to AV ≤ 10 magnitudes. For cloud depths above this threshold there is a notable break in the linear correlation. Although optically thick C18O emission could produce this departure from linearity, CO depletion in the denser, coldest regions of L977 may be the most likely cause of the break in the observed correlation. We directly derive the C18O abundance in this cloud over a broad range of cloud depths and find it to be virtually the same as that derived for IC 5146 from the data of Lada et al. (1994). Our results suggest that the use of C18O as a column density tracer in molecular clouds can lead to a 10 to 30% underestimation of overall cloud mass. In regions of very high extinction (AV > 10 mag), such as dense cores, our results suggest that C18O would be a very poor tracer of mass. We estimate the minimum total column density required to shield C18O from the interstellar radiation field to be 1.6 ± 0.5 magnitudes of visual extinction. Accepted by The Astrophysical Journal http://cfa-www.harvard.edu/∼jalves Spectral Indices of Centimeter Continuum Sources in Star-Forming Regions: Implica- tions on the Nature of the Outflow Exciting Sources Guillem Anglada1, Eva Villuendas2, Robert Estalella2,3, Maria T. Beltr´an2, Luis F. Rodr´ıguez4, Jos´e M. Torrelles1,5, Salvador Curiel4 1 Instituto de Astrof´ısicade Andaluc´ıa,CSIC, Apdo. Correos 3004, E-18080 Granada, Spain 2 Departament d’Astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, E-08028 Barcelona, Spain 3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 4 Instituto de Astronom´ıa,UNAM, Apdo. Postal 70-264, 04510 M´exico D.F., Mexico 5 Instituto Mediterr´aneode Estudios Avanzados (CSIC-UIB), Universitat de les Illes Balears, Carretera de Valldemosa km 7.5, E-07071 Palma de Mallorca, Spain (visitor; present address) E-mail contact: [email protected] 1 We present 6 cm VLA observations of nine regions with molecular or HH outflows, that appear to be driven by young stellar objects of low bolometric luminosity. Radio continuum emission at 3.6 cm had been previously detected toward the center of symmetry of 13 outflows in these regions. With the new 6 cm observations we have been able to estimate the spectral index in the 6 cm to 3.6 cm wavelength range for 11 of these outflow central (Class 0 and Class I) sources. All the spectral indices obtained for these central sources are positive, and consistent with partially thick thermal free-free emission. We discuss this result in relation to the current models for the origin of cm radio continuum emission in outflow sources. In particular, we find that the observed flux densities of these sources fit very well in the observational correlation between the radio continuum luminosity and the momentum rate of the outflow (e.g., Anglada 1996), and that the results are consistent with the central sources being thermal radio jets. Our data suggest that the ionized part of the jets begins at a distance ∼<10 AU from the star. In addition, we have derived a positive value for the spectral index of the Herbig-Haro object 32A, indicative of partially thick thermal emission. For the remaining sources detected in the nine fields observed, our results indicate that the emission is, in general, non-thermal as expected for background extragalactic sources. However, a few of these sources could be related to the star-forming regions studied. Accepted by The Astronomical Journal (to appear December 1998 issue) Preprint available at http://www.astrosmo.unam.mx/∼luisfr/ http://www.am.ub.es/∼robert/ISMpub.html ISO-SWS Observations of Herbig Ae/Be Stars: HI Recombination Lines in MWC1080 and CoD -42◦ 11721 M. Benedettini1, B. Nisini1, T. Giannini1, D. Lorenzetti2, E. Tommasi3, P. Saraceno1 and H.A. Smith4 1 CNR-Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere, I-00133, Roma, Italy 2 Osservatorio Astronomico di Roma, via Frascati 33, I-00040, Monte Porzio, Italy 3 ISO Science Operations Centre, Villafranca, P.O. Box 50727, E-28080 Madrid, Spain 4 Harvard - Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA E-mail contact: [email protected] ISO-SWS grating spectra are obtained towards the two Herbig Ae/Be stars MWC1080 and CoD -42◦ 11721, showing hydrogen recombination lines of the Brackett, Pfund and Humphreys series. The observed line decrements in each spectral series are consistent with emission from ionized winds, as expected from these early-type stars. We compare the observed line emission with a wind model assuming a constant rate of mass flow from the star, which allows to consistently derive mass loss rate and distance of both stars. We also show that the observed decrements can only be explained by assuming a ionization bounded compact regions whose sizes are a few tens of the stellar radii. Accepted by Astronomy and Astrophysics The Ionization Fraction in Dense Molecular Gas II: Massive Cores Edwin A. Bergin, Ren´ePlume, Jonathan P. Williams, and Philip C. Myers Harvard-Smithsonian Center for Astrophysics MS-66, 60 Garden Street, Cambridge, MA 021 38-1596, USA E-mail contact: [email protected] We present an observational and theoretical study of the ionization fraction in several massive cores located in regions that are currently forming stellar clusters. Maps of the emission from the J = 1 → 0 transitions of C18O, DCO+, + 13 + N2H , and H CO , as well as the J = 2 → 1 and J = 3 → 2 transitions of CS, were obtained for each core. Core densities are determined via a large velocity gradient analysis with values typically ∼ 105 cm−3. With the use of observations to constrain variables in the chemical calculations we derive electron fractions for our overall sample of 5 cores directly associated with star formation and 2 apparently starless cores. The electron abundances are found to lie within a small range, −6.9 < log10(xe) < −7.3, and are consistent with previous work. We find no difference in the amount of ionization fraction between cores with and without associated star formation activity, nor is any difference found in electron abundances between the edge and center of the emission region. Thus our models are in agreement with the standard picture of cosmic rays as the primary source of ionization for molecular ions. With the addition of 2 previously determined electron abundances for low mass cores, and even more massive cores associated with O and B clusters, we systematically examine the ionization fraction as a function of star formation activity. This analysis −8 demonstrates that the most massive sources stand out as having the lowest electron abundances (xe < 10 ). Accepted by the Astrophysical Journal Preprints available at http://cfa-www.harvard.edu/∼ebergin/ or by request: [email protected] Multi-lines analysis of the spectra of Herbig Ae/Be stars Jean-Claude Bouret and Claude Catala Laboratoire d’Astrophysique de Toulouse, CNRS UMR 5572, Observatoire Midi-Pyr´en´ees, 14 avenue Edouard Belin, F-31400 Toulouse, France E-mail contact: [email protected] We present the results of the study of four Herbig Ae/Be stars (AB Aur, BD+46◦3471, HD250550, BD+61◦154). For each of them, semi-empirical models of the structure of their winds have been constructed; we then calculated the Hα line, the Balmer discontinuity, as well as the C IV 1548 A˚ and Mg II h or k resonance lines. Thanks to comparison with observational data, we have deduced constraints on the parameters of the models, which provide us with a rather good representation of the averaged structure of the winds. Despite of the time variability of these stars, we show that we can gather trustworthy informations on the values of the mass loss rates of these stars. It is also shown that models with departure from spherical symmetry are needed to account for the observed shapes of the lines. In addition, radiative losses due to several transitions (lines and continua) of hydrogen have been evaluated, in order to gain more insight in the energy sources that generate the observed activity of these stars. Accepted by Astronomy & Astrophysics Preprints available from http://www.obs-mip.fr/omp/astro/people/bouret/publications.html The Structure and Emission of the Accretion Shock in T Tauri Stars Nuria Calvet1 and Erik Gullbring2 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA 2 Also Centro de Investigaciones de Astronomia, M´erida, Venezuela E-mail contact: [email protected] We have examined one of the key predictions of the magnetospheric infall model for Classical T Tauri stars (CTTS), namely, the formation of a shock on the stellar surface.